The present invention relates to a liquid crystal display device, and more particularly, it relates to a transmission/reflection type liquid crystal display device capable of display in both a transmission mode and a reflection mode.
Owing to their characteristics such as a small thickness and small power consumption, liquid crystal display devices are recently widely used in OA equipment such as a word processor and a personal computer, portable information equipment such as an electronic notebook and a camera-integrated VTR equipped with a liquid crystal monitor.
The liquid crystal display devices are divided roughly into two types: reflection type and transmission type. A liquid crystal display device is not a spontaneously luminous display device like a CRT (cathode-ray tube) and EL (electroluminescence), but light of a lighting device disposed behind a liquid crystal display panel (known as the so-called back light) is used for display in a transmission type liquid crystal display device and ambient light is used for display in a reflection type liquid crystal display device.
Since a transmission type liquid crystal display displays an image by using light from a back light, it can advantageously display bright images with a high contrast ratio without being affected by ambient brightness but disadvantageously consumes large power due to the back light included therein. A back light consumes approximately 50% or more of the power consumption in a general transmission type liquid crystal display device. Furthermore, in a very bright environment (for example, in the open air in fine weather), the visibility is degraded or the power consumption is further increased when the brightness of the back light is increased to attain sufficient visibility.
On the other hand, since a reflection type liquid crystal display device does not include a back light, it advantageously consumes very small power, but the brightness and the contrast ratio of display are disadvantageously largely affected by the environment such as ambient brightness. In particular, in a dark environment, the visibility is excessively degraded.
For the purpose of overcoming these problems, a liquid crystal display device having a function to display images in both the reflection mode and the transmission mode is disclosed in, for example, Japanese Laid-Open Patent Publication No. 11-101992.
This transmission/reflection type liquid crystal display device includes, in every pixel region, a reflection pixel electrode for reflecting ambient light and a transmission pixel electrode for transmitting light from a back light, so that images can be displayed with the mode switched between the transmission mode and the reflection mode or in the both modes. Accordingly, the transmission/reflection type liquid crystal display device has both the characteristic of a reflection type liquid crystal display device, that is, the small power consumption, and the characteristic of a transmission type liquid crystal display device, that is, bright display with a high contrast ratio minimally affected by ambient brightness. Moreover, the disadvantage of a transmission type liquid crystal display device that the visibility is degraded in a very bright environment (for example, in the open air in fine weather) can be suppressed.
The transmission/reflection type liquid crystal display device disclosed in Japanese Laid-Open Patent Publication No. 11-101992, however, has irregularities of the surface profile of the reflection electrode for increasing the brightness of display in the reflection mode (as is understood from, for example, FIGS. 6 and 9 of the publication). Accordingly, the thickness of a liquid crystal layer is largely varied in a reflection region, and hence, it is difficult to realize optimal display. Furthermore, it is necessary to precisely control the shape of the irregularities (convexes/concaves) for preventing light interference caused by the irregularities, which disadvantageously increases the fabrication cost.